Scan Tool Testing and Procedures

Terminal ``E'' or ``M''of the underdash ALDL connector allows access to a variety of system information known as ``Serial Data'' using SCAN type testers. Terminal ``E'' is used to transmit data on 1984-86 models and 1987 models that retain the 160 baud transmission rate ECM. Terminal ``M'' Is used on 1987 models with the 8192 baud transmission rate ECM. While using a SCAN tool to monitor serial data does not make use of diagnostic charts unnecessary, understanding what each position on the tester used measures and the function of the circuit involved can be useful in obtaining information that might otherwise prove time consuming or impossible to obtain using other test procedures.
Serial data access can be useful in obtaining information regarding intermittent malfunctions, as the SCAN tool can be installed and monitored while operating the vehicle under the conditions where the malfunction was reported to occur. If the malfunction seems to be related to specific functions that are monitored by the SCAN tool, the system can be checked in the applicable positions, under actual operating conditions. If the malfunction is not related to any specific area, the system can be monitored in all tester positions for a period of time or under a group of operating conditions. This allows the technician to observe the system and monitor changes in tester readings that may indicate a system malfunction.
Many diagnostic charts include provisions for the use of a SCAN type diagnostic tool to interpret serial data, and use of the SCAN tool is required to diagnose malfunctions on some 1987-88 models. The SCAN tool is capable of monitoring sensor inputs to the ECM and ECM commands to the various system operating elements, and provides the means to save time in diagnosis and prevent replacement of satisfactory components. Successful use of the SCAN tool for diagnosis requires an understanding of how the C-3 system functions, a thorough acquaintance with the tool manufacturer's recommendations and knowledge of the SCAN tool's limitations. Not all SCAN testers are compatible with every system. A SCAN tool that produces faulty readings should not be used, and problems should be reported to the tool manufacturer. Use of a faulty SCAN tool can result in improper diagnosis and unnecessary component replacement.
The SCAN tool can be used in 3 different modes: the normal or open mode; the 10K, special or ALDL mode; the factory test, back-up or 3.9K mode. Most SCAN tools are programmed so that the system will enter the special (ALDL, 10K) mode directly, and the `normal' or `factory test' modes must be selected manually, if available. In the special mode, all information incorporated into an engine/ECM combination is available. However, in the special mode engine operating characteristics are modified as follows: The closed loop timers are bypassed, EST spark is at full advance, the IAC (if equipped) controls idle speed at 1000 RPM, the park/neutral restrict functions are disabled, and on some engines the canister purge solenoid is activated. Although not all engine/ECM `families' are able to transmit information on the serial data line with the SCAN tool in the normal (open) mode, on engines that can be monitored in the open mode the SCAN tool allows data on certain operating parameters to be obtained without changing engine operating characteristics. Selecting the factory test or back-up mode causes the ECM to operate on the fuel back up logic programming, designed to allow system operation in the event of an ECM failure. While selection of this mode verifies that the back up programming is functioning properly, parameters that can be checked with the tool in this mode are of little value in diagnosing system malfunctions.
In the generally used special or normal test modes the SCAN tool allows a quick check of sensors and switches which provide inputs to the ECM, but on models with a 160 baud ECM, the data only updates once every 1.25 seconds making the tool less effective than a voltmeter for locating intermittent conditions which may last for less than 1.25 seconds. However, the SCAN tool does allow the technician to manipulate wiring harnesses or underhood components while observing the SCAN readout. This aids in locating intermittent malfunctions while the engine is not running.
It is also important to note that while the SCAN tool may indicate that a component has been activated by the ECM, this indication only represents the ECM command to the specific circuit. The only way to verify that the command has been transmitted and carried out is to observe operation of the specific component activated by the ECM, or by noting changes in the system that the device controls. However, the SCAN tool allows the technician to compare operating parameters of a poorly operating engine with the parameters of an engine which is operating properly. Therefore, system malfunctions that may not ordinarily set a trouble code in the ECM memory, such as a shift in sensor values or excessive resistance in an operating element, can be uncovered by comparing operation of like systems.
The SCAN tool is capable of testing a variety of electronic engine control systems, and therefore not all tester positions will be applicable for each engine/ECM combination being tested. If a position is selected that is not applicable to the particular combination, this condition will be indicated on the tester. Refer to the manufacturer's instructions and the following descriptions for information regarding the SCAN tester positions and how they can aid diagnosis.

A/C Request--A/C Clutch
The A/C Request position displays the status of the A/C signal line to the ECM. The A/C Clutch position displays ``on'' when the ECM energizes the compressor clutch.

Airflow
This display indicates air flow in grams per second (gm/sec). Readings should be the same as for the Mass Air Flow (MAF) sensor position (4-7 gm/sec) if no malfunction exists in the MAF circuit. If trouble code 33 or 34 is set, the reading will remain fixed at the value that the failure has detected.

BARO-Barometric Pressure
Barometric pressure is displayed as a voltage value of 2.5-5.5 volts, depending upon altitude and barometric pressure. BARO reading should be equal to MAP reading before engine is started.

Batt-AD Bat
Display reads battery voltage at ECM ignition feed.

BLM-Block Learn Memory Cell
There are from 2 to 16 different cells which the ECM learns at depending upon RPM and air flow. This parameter displays which cell the ECM is using for fuel calculations at the present time.

BPW-Base Pulse Width
Displays actual injector pulse time as calculated by the ECM in milliseconds (m/sec). Normal readings should be 1-4 m/sec and varying. Lower number indicates reduced fuel delivery, while higher number indicates increased fuel delivery. A fixed reading indicates open loop operation or a system malfunction.

CCP-Canister Purge
Display reads status of ECM command for canister purge solenoid. Reading of ``on'' indicates ECM signal to solenoid. On some systems, duty cycle is displayed from 0-100 percent.

Coolant Temp.
This position displays engine coolant temperature in degrees Centigrade. Can be used to detect malfunctions in the coolant temperature circuit if readings are not consistent with engine operation.

Crossover Count
The O2 sensor voltage fluctuates around a threshold voltage: the value of which is stored in the PROM. A counter within the ECM `counts' the number of times that actual oxygen sensor voltage crosses this threshold to indicate how fast the sensor is oscillating. The crossover count is the a value representing the number of times that the sensor value crosses this threshold in 1.25 seconds. A higher crossover count number indicates a more closely air/fuel mixture ratio.

Desired RPM
Indicates RPM to which ECM is trying to control idle speed.

Ign/Crank-Ignition Switch Status
The crank mode (on) position engages the starter. The SCAN displays the on/off status of the ignition switch as it is transmitted to the ECM.

EGR Signal
The SCAN tool monitors only the ECM drive command to the EGR solenoid. On models with Pulse Width Modulated EGR, reading is duty cycle of ECM signal from 0-100 percent. On models with EGR solenoid, ``on'' or ``off'' status of solenoid is indicated. Normal reading with engine running at idle should be 0 or off.

EGR Position
Indicates position of EGR valve pintle as voltage reading. Normal reading is 3-2 volts.

Exhaust
Indicates whether O2 sensor rich or lean operating condition. Normal displays are ``RH'' for rich indication and ``LN'' for lean indication.

Fan
Monitors ECM command to engine cooling fan. Displays ``yes'' when ECM energizes fan relay. Indication should be ``no'' when coolant temperature is below 106°C.

Fan Request
Displays status of A/C fan control switch. Should read ``yes'' when A/C fan switch is in on position or when A/C system pressure exceeds calibrated value; ``no'' at all other times.

IAC-Idle Air Control
The IAC is controlled by the ECM to maintain specified idle speed. The ECMcontrols the IAC in with voltage pulses (counts) and these counts are displayed in this position. The number of counts displayed represents the position the ECM ``thinks'' the IAC is in. Normal readings are in the 5-50 range.

Injector Pulse Width
Displays injector pulse width in milliseconds. Normal readings are 1-4 and varying.

INT-Integrator & BLM-Block Learn Memory
Normal readings for these positions are approximately 128. Higher readings indicate the ECM is adding fuel to the base calibration, while lower readings indicate the ECM is reducing fuel delivery. The integrator (INT) reading is short term correction and the readings vary when the ECM senses a need for fuel delivery correction. Block Learn Memory (BLM) is long term correction, and readings should only change if the integrator monitors a need for correction that lasts for a calibrated period of time. Normal BLM readings are 128-138.

Knock Retard
On 1984-86 engines equipped with ESC, this mode will indicate if the ECM is recognizing the retard signal from the ESC module and taking appropriate action. If knock is being detected, the numerical display on the tool will count up to 255, then start over.
On 1987-88 engines the display indicates the number of degrees the ECM is retarding the EST spark timing.

Knock Signal
Indicates knock sensor input to ECM. Display is ``yes'' when knock is detected and ``no'' when no signal received.

Loop Status (O/L--C/L)
This position indicates whether the engine control system is operating in open (O/L) or closed (C/L) mode. Most systems are designed to enter closed loop after a certain amount of run time, when the coolant temperature exceeds a specified value and the oxygen sensor begins to function. This mode can be helpful in detecting intermittent opens in the O2 sensor circuit, as the SCAN tool will indicate open loop operation long before the appropriate trouble code will be set in the ECM Memory.

LV8-Load Variable
Numerical representation of engine load, calculated by the ECM using RPM and air flow readings. MAF-Mass Air Flow Sensor

MAF-Mass Air Flow Sensor
Indicates the amount of air passing sensor in grams per second (gm/sec). Normal readings are 4-7 gm/sec. If code 33 or 34 is set, reading will display ECM default value.

MAP-Manifold Absolute Pressure
Indicates voltage signal from MAP sensor. Voltage readings are low when manifold pressure is low (high vacuum), and signals increase as manifold vacuum decreases. With ignition off and engine stopped, manifold pressure is equal to atmospheric pressure and voltage signals will be high (nearly 5 volts). With engine running, normal readings are 1-2 volts.
Comparison of sensor readings with readings from known good sensor can be used to isolate malfunctioning sensor. Sensors have a color code tab in the connector cavity, and sensors with the same colored tab have identical calibration. Readings from sensors with like identification tabs should be the same to within ±.4 volt.

MAT-Manifold Air Temperature
Indicates temperature of intake manifold air in degrees Centigrade. Reading should be close to ambient temperature when engine is cold, and reading should rise as engine and underhood temperatures increase.

MPH/VSS
Displays vehicle speed by sensing pulses transmitted to the ECM from the vehicle speed sensor.

O2 Sensor Voltage
Oxygen sensor voltage is displayed in millivolts (mv) from 1 to 999. If displayed voltage is consistently below 350 mv, the ECM is receiving a lean signal. If voltage is consistently above 550 mv, ECM is receiving a rich mixture signal. If reading remains fixed between 350 mv and 550 mv for a specified amount of time, trouble code 13 will set in ECM.

Park/Neutral Switch
Indicates status of park neutral switch. Indication may vary depending upon tool manufacturer. However, it is important that tool indicate change in status when gear selector is moved from park or neutral position to a drive range.

Power Steering Switch
Displays status of switch. Indication may vary depending upon tool manufacturer. However, it is important that tool indicate change in status when high power steering pressure exists such as when steering is held against stop.

PPSW
Indicates fuel pump feed circuit voltage. Should indicate battery voltage when engine ic cranking or running.

RPM (Engine Speed)
Displays engine RPM as transmitted to ECM from reference sensor. Can be used to detect extra reference pulses caused by electrical interference in reference circuit.

Spark Advance
Indicates varying degrees of spark advance.

TPS--Throttle Position Sensor Voltage
Values displayed will be voltage values through the TPS as sensed by the ECM. Value at idle should be as specified in adjustment procedure, while value at wide open throttle should be approximately 5 volts.

Throttle Angle
Indicates throttle position as percent. Reading should be low at idle and 100 percent at wide open throttle.

TCC-[1][2]Torque Converter Clutch
Indicates status of ECM command to energize TCC. Does not indicate that clutch is engaged, only that ECM has completed ground in clutch solenoid circuit.

Transmission/Transaxle Gear Position Switches
Indicates signal to ECM from switch. Indication may vary depending upon tool manufacturer. However, it is important that tool indicate change in status when gear is engaged.

Trouble Codes
Trouble codes set in ECM memory will be displayed on SCAN numerical readout.

VAC-Vacuum (Differential Pressure) Sensor
Indicates voltage through vacuum sensor. Voltage reading is low when vacuum is low and increases as vacuum increases. With ignition on and engine stopped, reading should be under 1 volt, and with engine idling (high vacuum) reading should be over 3 volts.
Comparison of sensor readings with readings from known good sensor can be used to isolate malfunctioning sensor. Sensors have a color code tab in the connector cavity, and sensors with the same colored tab have identical calibration. Readings from sensors with like identification tabs should be the same to within ±.4 volt.

Wastegate Actuator
Indicates duty cycle of ECM command to turbocharger boost pressure control as percentage.

WOT-Wide Open Throttle Switch
SCAN displays on/off status of WOT switch as sensed by ECM. Switch should be on when throttle is in wide open position.